When you play the card Ameba in face up attack mode does your opponent get 2000 points of damage or does that happen when your opponent takes control of your Ameba?

Your opponent will not immediately take damage when your summon "Ameba". It's effect will only inflict damage to your opponent when control of it switches to your opponent's side of the field while it is face-up. So if your opponent uses a card like "Brain Control", "Snatch Steal", "Mind Control", or "Change of Heart" and takes control of it while "Ameba" is face-up, they will take 2000 points of damage to their Life Points. You can use cards like "Creature Swap", "Shien's Spy", or "Mystic Box" to force your opponent into taking "Ameba" and losing Life Points. If your opponent uses a card like "Monster Reborn" to retrieve "Ameba" from your Graveyard, they will not take 2000 points of damage. The effect of "Ameba" will only work once while it is face-up ,so you will not take damage if you gain back control of your monster. However, if "Ameba" is flipped face-down by a card effect, like "Book of Moon", and flipped face-up, if another player takes control of the monster they will receive 2000 points of damage.